The printing techniques commonly used in the graphic arts industry, i.e., newspapers, books, etc., utilize an all-or-nothing inking procedure; i.e., ink is either deposited at each point on the printing surface or it is not. While this technique poses no problems when it is desired to print text, the printing of pictures, such as photographs, introduces the problem of printing the continuous tones in the gray scale ranging from black to white. The problem is generally solved by transforming the continuous tones of the original image into halftones. Halftone images are typically produces by a large number of inked dots whose size or spacing relative to one another tricks the eye into perceiving shades of gray rather than the individual dots. This technique works best when the largest dots and the spacing between the dots is small compared with the visual accuity of the eye. Early halftone generating systems used either variable sized dots with uniform spacing or uniform sized dots with variable spacing.
Electronic phototypesetting systems developed over the years have greatly increased the speed of type composition. Many such systems incorporate a halftone generating capability and in addition to storing typeface characters in a font memory also store halftone dot characters representative of discrete levels of the gray scale. In order to produce a halftone image, these systems scan the original image, sample the gray scale value at discrete intervals and convert these values to multibit binary numbers which are then used to access the stored dot character representative of that value from the dot character font memory. U.S. Pat. No. 3,806,641 to Crooks discloses a system of this type.
The data processing capabilities of such electronic phototypesetting systems have made possible a drastic reduction in the time necessary to prepare plates for printing material comprised of both text and illustrations. However, the resolution of halftone images produced by such sampled data systems generally falls short of that obtainable when non-digital techniques are employed.